This request is for the continuation of a project aimed at contributing to our understanding of enzymes catalysis by providing information on the steric course of a number of enzyme reactions at prochiral and pro-prochiral centers. The work does, for the most part, involve the use of isolated enzymes and substrates labeled stereospecifically with tritium and/or deuterium followed, if necessary, by stereospecific degradations of the products. Enzyme reactions to be investigated are a number of pyridoxal phosphate-catalyzed alpha, beta-elimination and elimination-addition reactions of amino acids, the transfer of methyl groups in biological C-, N-, O- and S-methylation reactions, the isoprenylation of aromatic ring systems and some enzyme reactions in which methyl groups are transformed into methylene groups or vice versa (citrate synthetase, isopentenyl pyrophosphate isomerase) or in which methylene groups are transferred (dCDP and dUDP hydroxymethylase).